The present invention relates to a radiation-polymerizable mixture containing a polymeric binder, at least one ethylenically polyunsaturated compound and a photopolymerization initiator, and also to a recording material containing the radiation-sensitive mixture.
Photopolymerizable mixtures for producing relief printing plates which are used, for example, in the flexographic printing process contain a polymeric binder, a compound polymerizable by free radicals and a photopolymerization initiator. When exposed, the mixture is crosslinked and becomes insoluble. The compounds polymerizable by free radicals are esters of unsaturated carboxylic acids, in particular of acrylic and methacrylic acid, with polyhydric alcohols which may optionally contain urethane or ether groups.
DE 2,215,090 (=U.S. Pat. No. 4,423,135) describes a mixture of a thermoplastic elastomeric three-block copolymer, a photopolymerization initiator and an ethylenically unsaturated compound containing at least one terminal, ethylenically unsaturated group. The mixture has, however, only a low thermal stability. This manifests itself, for example, in the fact that plates which are insoluble in solvents --even when a thermal polymerization inhibitor is added--are produced in the thermoplastic processing during the production of an unexposed printing plate and in the case of simultaneous exposure to shearing forces or of storage at relatively high temperatures. The quality of a relief printing plate produced from this radiation-sensitive mixture is therefore considerably reduced. In extreme cases a relief image can no longer be produced. The ethylenically unsaturated compounds containing at least one terminal ethylenically unsaturated group described in DE 2,215,090 are, in particular, (meth)acrylates such as, for example, 1,6-hexanediol diacrylate. These compounds are relatively readily volatile and are known to cause irritation of the eyes, the respiratory organs and the skin in man.
DE 2,815,678 (=U.S. Pat. No. 4,197,130) describes a photosensitive mixture of a thermoplastic elastomeric block copolymer, a photopolymerization initiator and at least one ethylenically unsaturated compound. The ethylenically unsaturated compounds include diesters of fumaric acid or maleic acid with monofunctional alcohols, for example, bis(2-ethylhexyl) fumarate. A great disadvantage of the mixtures which contain these unsaturated compounds is their low photosensitivity, in particular in comparison with mixtures containing hexanediol diacrylate. This is because 1,2-disubstituted ethylenically unsaturated monomers have a lower polymerization reactivity than 1,1-disubstituted ethylenically unsaturated monomers (T. Otsu et al., Makromol. Chem., Rapid Commun. 2: 725 (198) and T. Otsu, Makromol. Symp. 10/11: 235 (1987)). Furthermore, the photosensitivity of photopolymerizable mixtures depends on the functionality of the monomers: high crosslinking rates are known to be achievable only with polyfunctional monomers (C. G. Roffey, Photopolymerization of Surface Coatings, Wiley-Interscience, New York 1982, and H. Barzynski et al., Chemiker-Zeitung 96: 545 (1972)).